Slice-selective excitation with B₁⁺-insensitive composite pulses

Spatially selective excitation pulses have been designed to produce uniform flip angles in the presence of the RF and static field inhomogeneities typically encountered in MRI studies of the human brain at 7 T. Pulse designs are based upon non-selective, composite pulses numerically optimized for the desired performance over prescribed ranges of field inhomogeneities. The non-selective pulses are subsequently transformed into spatially selective pulses with the same field-insensitive properties through modification of the spectral composition of the individual sub-pulses which are then executed in conjunction with an oscillating gradient waveform. An in-depth analysis of the performance of these RF pulses is presented in terms of total pulse durations, slice profiles, linearity of in-slice magnetization phase, sensitivity to RF and static field variations, and signal loss due to T(2) effects. Both simulations and measurements in phantoms and in the human brain are used to evaluate pulses with nominal flip angles of 45° and 90°. Target slice thickness in all cases is 2mm. Results indicate that the described class of field-insensitive RF pulses is capable of improving flip-angle uniformity in 7 T human brain imaging. There appears to be a subset of pulses with durations ?10 ms for which non-linearities in the magnetization phase are minimal and signal loss due to T(2) decay is not prohibitive. Such pulses represent practical solutions for achieving uniform flip angles in the presence of the large field inhomogeneities common to high-field human imaging and help to better establish the performance limits of high-field imaging systems with single-channel transmission.     

Synthese de dienes-1,3 et de styrenes fonctionnalises par carbopalladation catalytique d'allenes

The palladium (O) catalyzed reaction of aryl and vinyl halides with allenes in the presence of sodium ethyl malonate leads with good yields and in certain cases with high stereoselectivity to styrenes and 1,3-butadienes derivatives.     

The Role of Specific Side Groups and pH in Magnetization Transfer in Polymers

The nature of water–macromolecule interactions in aqueous model polymers has been investigated using quantitative measurements of magnetization transfer. Cross-linked polymer gels composed of 94% water, 3%N,N′-methylene-bis-acrylamide, and 3% functional monomer (acrylamide, methacrylamide, acrylic acid, methacrylic acid, 2-hydroxyethyl-acrylate, or 2-hydroxyethyl-methacrylate) were studied. Water–macromolecule interactions were modified by varying the pH and specific functional group on the monomer. The magnitudes of the interactions were quantified by measuring the rate of proton nuclear spin magnetization exchange between the polymer matrix and the water. This rate was highly sensitive to the presence of carboxyl side groups on the macromolecule. However, the dependence of the rate on pH was not consistent with simple acid/base-catalyzed chemical exchange, and instead, the data suggest that multiequilibria proton exchange, a wide distribution in surface group pKvalues, and/or a macromolecular structural dependence on pH may play a significant role in magnetization transfer in polymer systems. These model polymer gels afford useful insights into the relevance of chemical composition and chemical dynamics on relaxation in tissues.     

Measurement and correlation of volumetric heat transfer coefficients of cellular ceramics

The volumetric heat transfer coefficients (h_v) between cellular ceramics and a stream of air were measured using the single-blow transient experimental technique in conjunction with an inverse analysis. Test specimen made of mullite, YZA, SiC, cordierite and cordierite with LS-2 coating was studied. The number of pores per centimeter (PPC) ranged from 4 to 26 and the specimen thickness ranged from 6 to 12 mm. Based on the experimental data, the volumetric heat transfer coefficients were generalized by developing Nusselt number vs. Reynolds number correlations of the form Nu_v=C Re^m for the materials studied. The effects of pore length-scale and specimen thickness on the volumetric heat transfer coefficients are presented and discussed.     

Biochemical data-processing with microcomputers : Part 3. On-line data acquisition from a continuous flow analyzer

A program, written in BASIC, is described which allows data acquisition from a continuous flow analyzer. The program was developed for a readily available microcomputer, but should be easily modified for use on similar machines. Once the peak height has been measured, the concentration of the analyte is calculated by reference to a previously defined calibration. The program is designed to handle data from more than one channel, although there is a practical limit of 3–4 simultaneously active channels. The results of the separate assays are collated and printed as a group for each specimen, even when the analytical methods require different times for completion.     

Regio- and enantioselective control in the reactions of alpha-(N-carbamoyl)alkylcuprates with allylic phosphates

Alpha-(N-carbamoyl)alkylcuprates (RCuCNLi or R2CuLi) react with allylic phosphates to afford homoallylic amines in good chemical yields. Regioselectivity is governed by steric factors in both the cuprate reagent and phosphate substrate and systems can be designed to give either the S(N)2' or S(N)2 substitution product cleanly. Excellent enantioselectivities can be achieved with either a scalemic alpha-di[(N-carbamoyl)alkyl]cuprate and an achiral phosphate or with a scalemic allylic phosphate and an achiral cuprate reagent. [reaction: see text]